submodule: prevent overwriting .gitmodules on path reuse

Adding a submodule at a path that previously hosted
another submodule (e.g., 'child') reuses the submodule
name derived from the path. If the original submodule
was only moved (e.g., to 'child_old') and not renamed,
this silently overwrites its configuration in .gitmodules.

This behavior loses user configuration and causes
confusion when the original submodule is expected
to remain intact. It assumes that the path-derived
name is always safe to reuse, even though the name
might still be in use elsewhere in the repository.

Teach module_add() to check if the computed submodule
name already exists in the repository's submodule config,
and if so, refuse the operation unless the user explicitly
renames the submodule or uses the --force option,
which will automatically generate a unique name by
appending a number (e.g., child1).

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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